There has been a demand for improvement in the operating frequency of electronic components paralleling the trend towards increasing the speed and capacity (greater frequency bands) in the field of mobile communication represented, for example, by the mobile telephone system. This also holds good with magnetic devices such as inductor transformers and their use in high frequency bands is now studied. Typical of such materials as expected for use in such high frequency bands is a nano-granular magnetic thin film of the type in which the grains of a magnetic material are included in an insulating material. The nano-granular magnetic thin film has a structure in which the magnetic grains of the order of a nano-scale (nm, 10−9 m) are enclosed by the grain boundaries of the insulating material. This is subjected to a wide study as a magnetic material for use in high frequency bands because of the expected reduction in crystalline magnetic anisotropy due to the improved fineness of magnetic grains as well as the improved electric specific resistance due to the grain boundaries of the insulating material.
For instance, Japanese Laid-Open Patent Publication No. 9-82522 proposes a magnetic film designed to obtain an uniaxial magnetic anisotropy of a proper magnitude and a permeability having excellent high frequency characteristics including a high electric resistance and a high degree of saturation magnetization. Also, Japanese Laid-Open Patent Publication No. 10-270246 proposes a magnetic film designed to obtain excellent soft magnetic properties in high frequency bands including an anisotropic magnetic field of over 20 oersted (Oe), an electric specific resistance value of 50 μΩcm or over and a saturation magnetic flux density of 16 kG or over. These nano-granular magnetic thin films employing CoFe alloy attract a notice in that they have a structure in which the magnetic grains of CoFe alloy crystals are enclosed by the grain boundaries of the insulating material composed of a ceramic and both a high saturation magnetization and a high electric specific resistance coexist with each other.
With such magnetic material used in high frequency bands, however, it is required to increase the resonant frequency of the permeability which limits its operating band. In order to increase the resonant frequency of permeability, it is required to have high saturation magnetization (hence high permeability) and electric specific resistance as well as an anisotropic magnetic field of a proper magnitude.
In the case of the foregoing conventional nano-granular films using CoFe alloy, however, a magnetic film is produced by reactive sputtering for effecting the film formation in an oxygen atmosphere. As a result, the metal in the magnetic grains and the metal forming the insulating material are oxidized. When the magnetic metal is oxidized, the saturation magnetization is decreased to lower the resonant frequency and eventually the operating band is decreased. In addition, no consideration is given to the fact that any excessive growth of the magnetic grains increases the crystalline magnetic anisotropy within the grains thus deteriorating the soft magnetic properties and that any excessively thin thickness of the insulating material deteriorates the resistivity and thus the eddy current loss is increased.
Further, while the previously mentioned nano-granular films are relatively high in resistivity than metallic magnetic films due to the structure in which the magnetic grains are enclosed by the insulating material, it cannot be said that they are sufficiently high in resistivity as compared with oxide magnetic material such as ferrite. Thus, in order to increase the resistivity of a magnetic film, it has been proposed to increase the ratio of an insulating material to the whole film or to use a multilayer film produced by alternately laminating magnetic layers composed of nano-granular films and insulating layers formed by metal oxide or the like. Then, while increasing the proportion of the insulating material in the nano-granular film as mentioned above has the effect of increasing the whole resistivity due to the increased thickness of the insulating material which separates the magnetic grains from each other, conversely the exchange interaction between the magnetic grains is deteriorated and the coercive force is increased due to an increase in the average distance between the adjacent magnetic grains. Also, the proportion of the magnetic grains is relatively decreased and thus the saturation magnetization is inevitably decreased. In the case of the magnetic film used at a high frequency, the saturation magnetization and the anisotropic magnetic field are both important physical quantities which determine the resonant frequency and therefore any attempt to attain an increased resistance by such method has a large possibility of deteriorating the high frequency characteristics.
Further, the above-mentioned multilayered magnetic film is such that while the resistivity is increased by the introduction of the insulating layers, capacitors are formed within the magnetic film because of its multilayered structure thus causing a loss due to a displacement current. In addition, the thickness of the insulating layers is relatively large ranging from 100 nm to several μm so that there is a disadvantage of decreasing the magnetic coupling between the magnetic grains holding the insulating layers therebetween thus eventually failing to ensure adequate magnetic properties.
The present invention has stemmed from the noticing of the foregoing deficiencies, and it is an object of the present invention to provide a magnetic thin film utilizing a granular film and having excellent high frequency characteristics, a method of producing the same, and a multilayered magnetic film and magnetic components and electronic equipment utilizing the same.